In electronic input scanners for document scanning, image information is acquired by sensing light from an image at an array of photosites arranged across a path of relative movement of the array and the image. The photosites, typically photodiodes or amorphous silicon sensors, are formed on a semiconductor substrate or chip, with a number of chips butted or otherwise arranged closely together to form the array. The photosite array may provide a 1:1 correspondence of photosites to the width of the actual image (a full width array), or may rely on optics to reduce the apparent image size to correspond to a smaller array. This array is usually a single chip. In use, a photosite produces an output signal proportional to light intensity detected at the photosite.
Responsivity at the photosites is measured against a standard value. Gain is a measure of sensitivity of the photosite to light and is the slope of the curve of light intensity (x-axis) versus output voltage (y-axis). Offset indicates the voltage output of the photosite at zero light intensity, or constitutes the y-axis intercept of the light intensity curve. These values have a tendency to vary somewhat from photosite to photosite within a range of values. Uniformity is desirable to avoid a streaking response.
A typical output signal from an array of photosites is offset from correct values by two types of offsets. The first type of offset is an output or line level offset, where the entire scan line of pixels is shifted with respect to a reference by some D.C. voltage level. There are numerous methods for correcting this offset, all of which apply the same D.C. correction factor to each pixel.
The second type of offset is a pixel-to-pixel variation in response, caused by several factors, the most important of which is photodiode leakage current. Typically, this offset value is small, e.g., a CCD photodiode having an integration time of 1 msec. may have an offset voltage of a maximum of 1 millivolt. However, if the maximum output signal is also low, e.g., under 300 millivolts, then this offset starts to become noticeable. This is particularly true if the CCD is operated under a wide range of temperature variations, since leakage current increases with increasing temperatures.
Pixel to pixel gain correction corrects for differences in pixel to pixel response due to non-uniformities in illumination profile, lens fall off, variations in CCD pixel to pixel response, etc. By correcting for these non-uniformities in the analog domain, the dynamic range of the system can be preserved, because each pixel will have the same dynamic range before being converted to a digital signal. Looking at the problem in another way, the closer that the zero value for the range of pixel values can be brought to the reference value (in this case, black), the finer the resolution to cover the range of possible pixel values to the maximum value (white).
U.S. Pat. No. 4,698,685 to Beaverson shows an arrangement which provides a gain correction for each pixel in an array. Gain values are stored for each pixel in an electronic storage device. As data is acquired by the array and directed to an image processor, each incoming pixel value is multiplied by a selected gain correction value to produce a gain corrected output. U.S. Pat. No. 4,639,781 to Rucci et al. shows that distortions in a video signal may be corrected by applying a continuous gain adjustment to the video information generated at the pixels and dynamically changing the gain factors on a line by line basis. U.S. Pat. No. 4,660,082 to Tomohisa et al., teaches that calibration and shading correction of image data may be corrected in synchronism with input scanning by comparison to a density reference value. U.S. Pat. No. 4,216,503 to Wiggins shows deriving offset and gain values from the sensor, storing those values and subsequently using those values for signal correction. U.S. Pat. No. 4,314,281 to Wiggins et al. teaches providing a compensation signal compensating for variations in light to which the sensors are subjected and deriving the compensation signal over a group of pixels, by taking an average response from the group as the group is exposed to a test pattern. U.S. Pat. No. 4,602,291 to Temes teaches a multimode pixel correction scheme which includes correction for pixel offset and gain. U.S. Pat. No. 4,920,428 to Lin et al. teaches the use of an attribute value in conjunction with a gain or offset correction to determine a shift of the correction values, thereby increasing the effective range of correction.